1. Field of the Invention
The invention relates to an arrangement for preventing the occurrence of current spikes during the charging of a capacitor.
More particularly, the invention relates to an electrophotographic copier in which the occurrence of current spikes is to be prevented during the charging of a flash capacitor which supplies the flash tube of a fusing station.
2. The Prior Art
Electrophotographic copiers are well known and require no detailed description for that reason. Briefly stated, the principle of operation of such apparatus is that an image carrier (plate, belt, drum) having a photoconductive surface is given a uniform electric charge over this surface and is thereupon exposed to the subject matter to be reproduced. This discharges the areas of the photoconductive surface in accordance with the radiation intensity which reaches them during the exposure, thereby creating on the surface a latent electrostatic image of the original being copied.
This image is then developed by bringing a toner powder into contact with the photoconductive surface, to which the toner powder clings electrostatically in a pattern corresponding to the electrostatic image. The thus developed image is then transferred to a support or copy carrier, e.g., a copy sheet. However, after such transfer the image must be fixed on the copy carrier, since otherwise it would simply rub off the same. This fixing is usually effected in a fusing station where the toner particles making up the image are heated so that they soften and coalesce, becoming sticky and firmly adhering to the copy carrier.
When heat fusing is employed, the heat can be supplied in various ways. The older approach was to use electrically heated resistance elements (e.g., U.S. Pat. No. 2,965,868 to Eichler); however, in higher-speed copiers (i.e., copiers capable of making large numbers of copies per unit time) the requisite heat is now often supplied by flash tubes. These are gas-discharge tubes which produce a high-intensity, short-duration flash of light, and in so doing, liberate the requisite heat energy. The high energy required for producing the flash in supplied from a flash capacitor which must, of course, have a correspondingly high energy storage capability. Since these arrangements are used in high-speed copiers, it is important that after each discharge the capacitor is rapidly recharged. This is where problems occur.
In the prior art the flash capacitor is connected to the current-supply net. When the flash capacitor begins to recharge (after a previous discharge), strong current spikes or peaks occur which tend to overload the supply net. Since this is undesirable it has been proposed to use a phase angle control circuit which reduces the draw of charging current to the flash capacitor, to dampen out the peaks.
However, when a high flash frequency is required (in keeping with the making of a great number of copies per unit time), a phase angle control circuit cannot be satisfactorily used in conjunction with a low-ohmic current supply net. The reason is that the time available for each individual charge episode of the flash capacitor is not well used in these circumstances, so that excessively high current peaks develop during the effort to charge the capacitor within the available time to the high energy level which is required to cause sufficient heat emission from the flash tube for fusing of the toner.